Labour has cruised to a comprehensive win in the Christchurch East by-election.

Former social worker Poto Williams, who moved to the area from Auckland only nine months ago, was regarded by some as a surprise pick to contest the seat for Labour but staged a strong campaign, spearheaded by former long-serving Christchurch MP Jim Anderton.

She led from the outset of the votes being counted, and when final results were declared at 8.15pm she had a commanding 8119 votes to Matthew Doocey's 3506 for National.

The Greens' candidate, David Moorhouse, was back on 926 while the Conservatives' Leighton Baker received 487 votes.

Williams will replace Lianne Dalziel, who quit this year to successfully run for the Christchurch mayoralty. Dalziel held a 5334-vote majority after the 2011 election but National surprised Labour by comfortably winning the party vote, capturing 46 per cent compared to Labour's 31.5 per cent.

Christchurch East was the electorate hardest hit in the Canterbury earthquakes - an estimated 10,000 people were forced to leave because of damage to their homes and businesses.

It was previously regarded as a safe Labour seat, but the population changes prompted Labour to campaign all-out.

The party brought in the seasoned Anderton to steer the campaign and leader David Cunliffe was a regular visitor, campaigning alongside Williams.

Other senior members of the party showed up during the campaign to lend their support.

Labour has held the seat since it was formed in 1922.

Williams, 51, has had a career in social and community services, largely in Auckland.

Wellington-born and Auckland-raised, the Cook Islander grew up in Ponsonby.

She has worked for the Ministry of Education and several social agencies including BirthRight, Healthcare NZ and some disability groups.

She moved to Christchurch after being appointed regional manager of youth agency St John of God Hauroa Trust in February until resigning in September when she was picked as Labour's candidate.

An emotional Williams dedicated the win to her late brother-in-law, Aaron Marsters, and thanked her entire family during a speech to her supporters at the Chisnallwood Intermediate School.

She heaped praise on Anderton, describing him as a "tireless" campaign manager who had managed to get the maximum out of everyone involved.

Cunliffe called the victory "a stonking win" and a signal that the National Government was "in serious trouble", both in Christchurch and around New Zealand.